Database storage architecture

ABSTRACT

Methods, systems and program products for database storage. In one implementation, data of a projection of a database is stored at least partly in grouped ROS format and partly in column format based on patterns of updating the projection data. The projection data is updated so that the updated projection is stored partly in grouped ROS format and partly in column format.

BACKGROUND

The present disclosure relates to a database storage architecture.

FIG. 1 illustrates row-store and column-store representations of a database table. A logical database design usually comprises one or more conceptual tables such as FIG. 1's table 120. A table is characterized by a set of attributes, usually depicted as column headings, for which each record in the table has respective (albeit potentially empty) values. In table 120, for instance, the attributes are Emp# (employee number), Dept (department), Room, Phone, and Age. In row-store arrangements values of a given record's successive attributes are typically stored physically adjacent to one another, as FIG. 1's depiction of row-store 122 suggests. In a column-store, as column-store 124 depicts, successive storage locations are occupied by different records' values of the same attribute.

A physical database design choice for both row-stores and column-stores can be the order in which rows or columns are stored. For example, FIG. 1's table 120 may be ordered in a row-store on attribute Room, in which case the first row in the table (Room=101) would be followed by the third row (Room=105) followed by the second row (Room=203). The choice of sort order can influence both query and update performance. For example, if table 120 is physically sorted on Room, then queries that include predicates on Room can be evaluated typically without scanning the entire table.

SUMMARY

In general, one aspect of the subject matter described in this specification can be embodied in a method that includes storing data of a projection of a database at least partly in grouped ROS format and partly in column format based on patterns of updating the projection data; and updating the projection data so that the updated projection is stored partly in grouped ROS format and partly in column format. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.

These and other aspects can optionally include one or more of the following features. Updating can further comprise storing rows of data for columns of the projection persistently. A time when the rows are stored can be based on at least one of an age of the rows or an aggregate size of the rows. Updating can further comprise storing rows of data for columns of the projection in a write-optimized store. Updating can further comprise estimating a storage size of rows of projection data to be stored based on a size of previously stored projection rows; and storing the rows in grouped ROS format or in column format based on the estimated storage size. The projection data that is stored in grouped ROS format can comprise one or more rows of a first set of the projection's columns. Each row's first columns can be stored together in a single file with at least one other row's first columns. The projection data that is stored in column format can comprise one or more rows for one or more first columns of the projection. Each row's first columns can be stored in separate files and each separate file stores the same first column from more than one row. Updating can further comprise storing rows of projection data for columns in grouped ROS format based on a frequency of use of the columns of the rows. Updating can be performed in response to a request to write data to the projection. Updating can be performed in response to processing a database operator to write data to storage.

In general, another aspect of the subject matter described in this specification can be embodied in a method that includes selecting at least two different storage formats for data of a database projection based on patterns of updating data of the projection; and updating projection data based on the patterns using the selected storage formats. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.

These and other aspects can optionally include one or more of the following features. The storage formats can comprise at least two of a grouped ROS storage format, a column storage format, and a write-optimized storage format. Updating can further comprise storing rows of the projection data of columns of the projection persistently. The rows are stored at a time based on at least one of an age of the rows or an aggregate size of the rows. Updating can further comprise storing rows of the projection data for columns of the projection in a write-optimized store. Updating can further comprise estimating a storage size of rows of the updated projection data based on a size of previously stored projection rows; and storing the rows of the updated projection data in grouped ROS format or in column format based on the estimated storage size. The updated projection data can be stored in grouped ROS format and wherein the updated projection data comprises one or more rows for a first plurality of the projection's columns. Each row's first columns can be stored together in a single file with at least one other row's first columns. The updated projection data can be stored in column format and wherein the updated projection data comprises one or more rows for one or more first columns of the projection. Each row's first columns can be stored in separate files and each separate file stores the same first column from more than one row. Updating can further comprise storing rows of the projection data in grouped ROS format based on frequency of use of columns of the rows. Updating can be performed in response to processing a database operator to write data to storage.

In general, another aspect of the subject matter described in this specification can be embodied in a method that a computer readable medium storing a projection of a table of a database system, the projection comprising columns of data of the table, the projection comprising: first rows for at least two of the columns in grouped ROS format, each of the first row's columns being stored in a single file with the columns of at least one other first row; and distinct second rows for at least two of the columns in column format, each of the second row's columns being stored in separate files and each separate file storing the same column of more than one of the second rows.

In general, another aspect of the subject matter described in this specification can be embodied in a method that includes associating at least two columns of a projection of a database system with respective storage locations, the method comprising ranking storage locations according to their respective speeds; ranking the columns according to their preferred speeds; and for each of the columns and according to their ranked order, assigning the column to the fastest storage location based on free space of the storage location and a size of the column. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.

These and other aspects can optionally include one or more of the following features. Ranking the columns can further comprise identifying one or more first columns in the columns that are in a sort order for the projection; and assuming a faster preferred speed for the first columns than other columns when determining an order for the columns. If the columns are to be stored in grouped ROS format, the preferred speed for each of the columns can be an average of all the columns' preferred speed. If the columns are to be stored in grouped ROS format, the preferred speed for each of the columns can be a fastest preferred speed of the columns.

In general, another aspect of the subject matter described in this specification can be embodied in a method that includes determining that a plurality of new rows for a plurality of a projection's columns are to be stored in persistent storage wherein the determining is based a criterion, and wherein the projection's columns store data for a table in a database system; selecting a storage format for the new rows wherein the storage format is grouped ROS format or column format and wherein grouped ROS format stores a plurality of the projection's columns together in a single file and wherein column format stores columns of the projection in separate files; and storing the new rows in the selected format in persistent storage. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.

These and other aspects can optionally include one or more of the following features. The criterion can be an age of the new rows. The criterion can be a total size of the new rows. Selecting a storage format can be based on an assessment of existing storage allocated to the table's data such that some but not all of the table's data is always stored in grouped ROS format.

In general, another aspect of the subject matter described in this specification can be embodied in a method that includes selecting a plurality of candidate projection containers wherein each candidate container holds rows for one or more columns of the projection; identifying a plurality of containers to merge in the candidate containers; selecting a storage format wherein the storage format is grouped ROS format or column format and wherein grouped ROS format stores a plurality of the projection columns together in a single file and wherein column format stores columns of the projection in separate files; merging the identified containers into a new container according to the selected storage format; and wherein selecting a storage format is based on an assessment of existing storage allocated to the projection such that some but not all of projection data is stored in grouped ROS format. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.

These and other aspects can optionally include one or more of the following features. The selected candidate containers can be in a common size range. The identified containers can be each associated with a start epoch that is smaller than the remaining candidate containers.

In general, another aspect of the subject matter described in this specification can be embodied in a method that includes determining that data is to be added to a projection of a database based on at least one of a size or an age of the data; and storing the data in at least partly in grouped ROS format and partly in column format. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.

Particular aspects of the subject matter described in this specification can be implemented to realize one or more of the following advantages. Beneficial elements of a row-oriented storage format are incorporated into a column-store Database Management System (DBMS) and a column query engine to obtain superior performance. The DBMS automatically learns the access patterns of user applications and employs the best storage format. The DBMS architecture improves the efficiency of disk access and hence improves the performance of analytic queries. The performance characteristics of the underlying storage device are taken into consideration and therefore can be optimized for the specific usage patterns of the user application. For example, single record retrievals and updates can be processed using the row-oriented disk format, whereas large retrievals can be processed using a columnar or a hybrid format. Similarly, often used columns can be placed on faster disks or faster tracks on the disk and less used columns can be placed on slower disks or slower tracks on the disk. Another use of this mechanism is to automatically archive or write-protect data to Write Once, Read Many (WORM) storage for compliance purposes.

The DBMS automatically learns the access patterns of user applications and employs the best storage format. Explicit overrides of these behaviors are allowed by the user. A combination of in-memory and on-disk storage structures is used to optimize the performance of data loads and queries. Techniques are presented for moving data from memory to disk structures to optimize performance. Moreover, DBMS data in memory continuously trickles to disk a benefit of which is that data does not need to be loaded to disk at night.

The architecture of the system is such that columns do not have to be stored separated from one another on disk. Specifically, columns that are often accessed together may be stored together, when requested in the by an end-user. When columns are stored together, additional compression types to take advantage of this configuration will be available. For example, in stock ticker data, the bid and ask prices are closely related, and can be compressed relative to each other for space savings. Another benefit is that tables having a very large number of columns (e.g., tens of thousands) can be stored by grouping sets of columns into single files in order to reduce the number of files that need to be open in order to process queries.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates grouped ROS and column representations of an example database table.

FIG. 2 is a schematic diagram of an example DBMS.

FIG. 3 illustrates global and local catalogs for an example system including a cluster of nodes.

FIG. 4 illustrates an example WOS storage block.

FIG. 5 illustrates a logical layout of grouped ROS and column formatted ROS containers for an example projection.

FIG. 6 illustrates an example index for a column formatted ROS container.

FIG. 7 illustrates an example index for a grouped ROS formatted ROS container.

FIG. 8A is a flow chart of an example technique for writing data to a grouped ROS formatted ROS container.

FIG. 8B illustrates writing data to a grouped ROS formatted ROS container.

FIG. 9A is a flow chart of an example technique for reading data from a grouped ROS formatted ROS container.

FIG. 9B illustrates reading data from a grouped ROS formatted ROS container.

FIG. 10 is a flow chart of an example technique for determining the format of a new ROS container for auto or hybrid optimized projection based on updating patterns for the projection.

FIG. 11 is a flow chart of an example technique for assigning columns to storage.

FIG. 12 illustrates example merge strata for a merge-out operation.

FIGS. 13A-B illustrate an example technique for determining the strata for a projection.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

This specification describes the architecture of a DBMS for performing analytic queries on very large quantities of data. In this architecture, the performance of the DBMS can be optimized based on a combination of grouped ROS and column storage formats with knowledge of the performance characteristics of the underlying storage devices, for example. An implementation of this system is the FLEXSTORE™ system, available from Vertica Systems, Inc. of Billerica, Ma.

FIG. 2 is a schematic diagram of an example flexible column store system 200. The database software executes on one or more computing devices, referred to as a nodes (e.g., 202). By way of illustration, a computing device comprises one or more personal computers or workstations, rack-mounted computers, or special-purpose computers, for instance. Data for a table is physically stored in one or more projections that comprise subsets of columns from the table. The node 202 includes one or more disks for storing data such as disks 204 and 206 and an in-memory, write-optimized store (WOS) 210 which stores, in a row-wise fashion, data that is not yet written to disk. The WOS 210 acts as a cache. In some implementations, the node 202 includes a WOS 210 for each projection.

Projections are stored on disk in a read-optimized store (ROS). The ROS comprises of one or more ROS containers. (ROS containers are discussed further in reference to FIG. 5 below.) Data is moved out of the WOS 210 to one or more ROS containers periodically or if the data exceeds a certain configurable age or size threshold, for example. A ROS container stores one or more columns for a set of rows of a projection in column format (e.g., 204 a, 204 b) or grouped ROS format (e.g., 204 c). Other beneficial ROS formats are possible. In column format, a given column's data can be retrieved from a ROS container separately from other columns' data. In grouped ROS format, some portion of data for all the columns stored in the grouped ROS will be retrieved from the ROS container when any of the other columns in the ROS container are accessed. In further implementations, empirical performance data and other performance related information about disks is used to intelligently place ROS containers in order to achieve better performance. (ROS container placement is discussed further in reference to FIG. 11 below.) By way of illustration, a disk 204 having slow access times and a large amount of storage space holds information from ROS containers 204 a-c whereas a small disk 406 with faster throughput rates stores ROS containers 206 a-b.

The system 200 illustrates two common operations performed on a database: a query (e.g., a SQL query) and a data load. The solid arrows illustrate the progress of a query 220 and the direction of data flow during a query processing. An end-user or an executing software program submits the query 220 to the system 200. The query 220 is parsed by a SQL parser 214 and provided to a query optimizer 214 which chooses the best projection(s) 224 for the query by referencing the catalog 208. Catalogs are discussed further in reference to FIG. 3 below.

A resulting query plan 226 is provided to an execution engine 216 which interacts with a Storage Access Layer (SAL) 218 to retrieve the data 228 responsive to the query 220 from the WOS 210 or the ROS, or both, and processes the data, before returning result records 222 back to the end user. The SAL 218 provides read and write access to WOS 210 and ROS storage. In some implementations, the execution engine 216 is a column-optimized engine designed to process data 228 in columns rather than in a row-by-row fashion. In yet further implementations, the execution engine 216 is capable of processing data 228 in its encoded form without first decoding the data.

An asynchronous background process known as the Tuple Mover 212 is responsible for migrating data from the WOS 210 to ROS containers (e.g., 204 a-c) and for merging ROS containers. The Tuple Mover 212 is discussed further below in reference to FIG. 12. The hatched arrows illustrate the direction of processing and data flow during a data load. Data records 234 are loaded into the WOS 210 and eventually the Tuple Mover 212, based on its policy (as described below) will sort, compress and encode the data into either a column or a grouped ROS formatted ROS container 230. In some implementations, the Tuple Mover 212 will also select an appropriate disk location (as described below) to use to store each ROS.

FIG. 3 illustrates global and local catalogs for an example system including a cluster of nodes. (In some implementations, the system includes only a single node.) Each node in a cluster has a global catalog 300 and a local catalog 302. The global catalog 300 stores information about the projections and their columns across the entire cluster, including user, schema, table, and projection information, and is accessible from all nodes of the cluster. The local catalog 302 stores information that is known only to a specific node (e.g., 202) within the cluster, such as the ROS containers and their storage locations on the node, for instance. The SAL 218 provides access to WOS and ROS storage on the node. In some implementations, each projection has a WOS on each node that the projection's data is stored on. Likewise, all columns of a projection are stored in ROS containers on nodes where the data is present, each node having different sets of rows.

By way of illustration, the global catalog 300 includes information identifying two projections: 306 and 314. According to the global catalog 3, the 306 projection has three columns which are each represented by a ProjColumn data structure (308 a-c) whereas the 314 projection has two columns each of which is represented by a ProjColumn data structure (316 a-b). If data in a projection is available on a specific node in the cluster, the local catalog 302 for that node identifies how to access this data in columns of the projection. In this example, the columns 308 a-c and 316 a-b can be accessed through a specific node and, as such, the local catalog 302 includes SALColumn objects 310 a-c and 318 a-b representing the columns 308 a-c and 316 a-b, respectively. Each SALColumn object is associated with a local catalog Wos data structure. For example, the column identified by the SALColumn object 310 a is associated with the local catalog Wos data structure 312 a for that column.

The SAL 218 provides access to a WOS data structure 322 which stores a WOSProj object (e.g., 324, 326) for each projection (e.g., 306, 314), and provides a mapping from the local catalog Wos data structures (e.g., 312 a) to WOSProj objects (e.g., 324). In various implementations, a WOSProj stores the information detailed in TABLE 1. Other information can be stored, however.

TABLE 1 WOSPROJ INFORMATION DESCRIPTION Committed Data In various implementations, committed data is stored as a vector of so-called WosBlocks. A committed WosBlock's number is its position in the committed vector. A WosBlock 400 is a storage block consisting of fixed 402 and variable 408 data as shown in FIG. 4 Fixed implies that all values for grouped ROSd contiguously, from the start of the block. Strings and other variable width values reference the variable section 408. Variable implies data is allocated from bottom of block upward. A free pointer to the bottom of a free space area 404 is maintained. This allows variable length strings to be stored efficiently. Uncommitted Data In various implementations, uncommitted data is stored as a map of transaction identifiers to transaction information. Uncommitted WosBlocks for a transaction are positioned in the WOS after all committed WosBlocks. In various implementations, transaction information comprises of a vector of WosBlocks 400 and save points. Save points can be created explicitly by a user or implicitly at the beginning of each new SQL statement so that upon error the transaction state can be rolled back to as if the statement never happened. As referred to herein, uncommitted[T] refers to the WosBlock in the uncommitted vector having the transaction identifier T. Save points are a map of save point identifiers to WOS positions. A WOS position is a 64 bit value - the 4 high bytes identify a WosBlock number and the low 4 bytes provided an offset within the block. Other types of WOS position representations are possible. Epochs In various implementations, there is a map between epochs and WosBlock positions referred to as an epoch map. An epoch is a monotonically increasing number that serves as a measure of time. In certain implementations, each transaction that occurs in the system will commit in a particular epoch. The epoch is advanced automatically every n minutes where n is configurable. By way of illustration, with 2³² blocks in the WOS at minimum size of 4 Kilobytes each, the WOS occupies 16 Terabytes of memory. Assuming we allocate blocks to ensure that at least 2 rows fit in a block, the maximum row size is 1 Gigabyte. Other sizes of the WOS are possible. Start Start information indicates the amount of unused storage for the first committed WosBlock. End End information indicates the WOS position of last row in the committed WosBlock vector.

In different implementations, objectives of the WOS are to stay within memory use limits, use memory efficiently, and tolerate fragmentation (e.g., allocating lots of 4 Kilobyte WosBlocks should not prevent allocating 8 Kilobyte WosBlocks, as long as overall memory use is within an acceptable limit). In some implementations, a WosBlock allocator process uses a virtual memory system on a node to handle fragmentation. By way of illustration, assume a 64 bit microprocessor (with minimally a 48 bit virtual address space). The WosBlock allocator maintains a set of regions: one per WosBlock size it supplies. Each region allocates a virtual memory region the size of a maximum WOS memory footprint (e.g., 25% of physical memory). Each region allocates WosBlocks of a single size, using a bitmap to track free blocks, for example. Whenever a WosBlock is referenced, the operating system finds a free page of physical memory and maps it into virtual memory. Whenever a WosBlock is freed by the WosBlock allocator, the allocator calls into the operating system (e.g., by invoking Unix memory functions such as mmap or madvise) and frees the physical pages associated with the WosBlock. The result is that the WOS occupies a large virtual address space, but the physical pages backing it do not exceed the WOS memory limit. By way of further illustration, for a 32 bit microprocessor the memory address space is limited to 32 bits (4 Gigabytes), thus making the address space and size of memory reasonably equivalent. In some implementations, the WosBlock allocator downgrades to a mechanism where it divides up the WOS memory evenly among the regions, resulting in an exponential scheme (e.g., twice as many 4 Kilobytes blocks as 8 Kilobyte blocks).

Because all information about a row in the WOS is shared across all columns of a projection, it is easy to annotate row-wise information. For example, the rows that correspond to each statement within a transaction can be used to roll-back changes made by a particular statement, or to implement the save points within the DBMS. In some implementations, the changes made to the WOS (e.g., inserts, updates and deletes) are annotated with a particular transaction or statement identifier, or save point name, as the case may be. When the system is requested to rollback the statement or transaction entirely to a specific save point, the relevant changes can be easily identified and removed from the database.

In various implementations, the WOS supports operations as detailed in TABLE 2. Other operations are possible, however.

TABLE 2 OPERATION DESCRIPTION Read In various implementations, the read operation causes the acquisition of references to all appropriate committed WosBlocks and uncommitted WosBlocks for the current transaction. In addition, start and end WOSProj information is cached. Write In various implementations, the write operation causes a row to be written as a unit to a WosBlock for an uncommitted transaction as follows: Compute space required: fixed size for row, plus size of strings; If the row will fit in the last WosBlock for the uncommitted transaction, store the row there (the last WosBlock is the last block used by the previous statement in this transaction, which may not be completed filled up yet); Otherwise, allocate a new WosBlock of size 2^(k), sufficient to store at least two rows of the current size, append the WosBlock to the uncommitted transaction's WosBlock list, and store the row in the newly allocated WosBlock; and Allocate WosBlock positions for the new WosBlock based on how many rows could fit (1 plus the number of minimum-size rows that can fit in the remaining space). Because subsequent rows can be packed into this WosBlock, enough sequential positions are allocated for a maximum number of rows. In this scheme, the amount of wasted space in the WOS can be bound to 50%, for example. There are two scenarios as follows: rows of size (2^(k)) + 1 result in WosBlocks of size 2^((k+2)): 50% wasted; or rows of size 2^(k) alternating with rows of size 1: 50% wasted. Other ways of performing the write operation are possible. Commit In various implementations, the commit operation causes data to be copied from uncommitted[T] to the committed vector. If the commit epoch is not in the epoch map (e.g., first commit in new epoch), all WosBlocks from uncommitted[T] are appended to the committed vector and the WosBlock index of first appended block is added to epochs. Otherwise, the last WosBlock of committed from blocks of uncommitted[T] is packed, starting with last uncommitted row and moving backwards. Once the last WosBlock of the committed vector is full, remaining blocks of uncommitted[T] are appended thereto. This process copies at most 1 block of rows. This design can lead to densely packed blocks. In some implementations, deletes are handled by noting the positions of the rows being deleted. Packing results in changes to the positions of the packed rows. If a transaction inserts and deletes a row in the same transaction, the delete position may be made obsolete by packing. At commit time, the deleted positions can be recomputed. A row that is inserted and deleted by a transaction cannot be observed after the transaction commits. Thus, it is safe to actually delete such rows when the transaction commits. In some implementations, the WOSProj scans for deleted uncommitted values, prevents their being copied into the committed section, and thereby implicitly removes these entries. Rollback Discard uncommitted[T]. Set Savepoint Insert into the savepoint map the given savepoint identifier paired with the relative position of the last row of the given transaction. The position is relative to the beginning of the blocks of uncommitted[T], rather than the whole WOS projection. Rollback to The savepoint identifier is looked up in savepoint position map, any blocks Savepoint in uncommitted[T] past that position are discarded and the count and free pointers in the last WosBlock are updated. In addition, all savepoints with higher identifiers are discarded from the position map (they no longer point to valid positions).

For a given node, there may be multiple ROS containers on disk storing some or all of a projection's data. The Tuple Mover process 212 depicted above in reference to FIG. 2 is responsible for migrating data from the WOS to ROS containers and for merging ROS containers. As a prelude to discussing the Tuple Mover process in more detail (see FIG. 12), the storage of data in ROS containers is described.

FIG. 5 illustrates a logical layout of grouped ROS and column formatted ROS containers for an example projection. In various implementations, a projection 700 is stored on disk as one or more ROS containers (e.g., 502, 504) in grouped ROS or column format. In further implementations, at least one of the ROS containers for the projection 700 is always in grouped ROS format. In this illustration ROS container 502 is in grouped ROS format and ROS container 506 is in column store format. The ROS container 502 is stored as a disk file 504 which comprises one or more blocks for a set of two or more columns 1 . . . N of the projection. Each block in a grouped ROS formatted ROS container holds data for some number of rows of the set of columns. (Note that a block is a logically contiguous chunk of the data and may or may not correspond to physical disk or file system block.) Block 504 a contains a first set of rows for the set of columns, for instance. Block 504 b contains a second set of rows for the set of columns. And block 504 c contains a third set of rows for the set of columns.

The ROS container 506 is in column format. ROS containers in column format comprise one or more files where each file in the ROS container stores data for a different column of a projection. In this illustration the ROS container 506 comprises disk files 508, 510 and 512, each of which holds one or more blocks of data for a single column of the example projection. File 508 comprises blocks 508 a-c. Block 508 a holds attributes for a first number of positions for column 1. Block 508 b holds attributes for a second number of positions for column 1. And block 508 c holds attributes for a third number of positions for column 1. Likewise, file 510 comprises blocks 510 a-c which hold attributes for a number of positions of column 2. And, file 512 comprises blocks 512 a-c which hold attributes for a number of positions of column N.

FIG. 6 illustrates an example index for a column formatted ROS container. An index is a position index used to tie data from different columns together. The position index 600 includes entries for each ROS container block. In a column formatted ROS container, each column's data is stored in separate file as a sequence of blocks and each file has its own index. In this example, column Col. 1's data is stored in blocks 602, 606, 608 and 610, and these blocks are indexed by index 600. (There is an implicit position number associated with records within columns.) An index stores entries relating the starting position number and a file offset of a block containing the data for the position. For example, index entry 600 a indicates that positions 0 through 999,999 for column 1 are stored in block 602, entry 600 b indicates that positions 1,000,000 through 1,999,999 are stored in block 606, and so on. Likewise column Col. 2's data is stored in blocks 614, 616, 618 and 620, and these blocks are indexed by index 612. Index entry 612 a indicates that positions 0 through 9,999 for column 2 are stored in block 614, entry 612 b indicates that positions 10,000 through 19,999 are stored in block 766, and so on. In order to access position 645 column 2, for example, the index 612 is first checked to determine the offset of the block that contains the position. Index entry 612 a indicates that block 614 contains position 645. Block 614 is then scanned to locate the position data within the block. Note that if only Col. 2's data is needed for a query, then neither the index nor the blocks for Col. 1 are touched. All column values for a specific row in a table can be retrieved by looking up each column's position index in turn—thus the position indexes serve to connect different columns across multiple files. Other block indexes are possible.

FIG. 7 illustrates an example position index for a grouped ROS formatted ROS container. The index 700 includes entries relating a starting position (e.g., a row) stored in the ROS container to a file offset of the block that stores the row. For example, index entry 700 a indicates that positions 0 through 9,999 are stored in block 702, entry 700 b indicates that positions 10,000 through 19,999 are stored in block 704, and so on. In order to access position 15,324, the index 700 is first checked to determine the file offset of the block that contains the position. Index entry 700 b indicates that block 704 contains position 15,324. Block 704 is then scanned to locate the position data within the block. Other block indexes are possible. In the grouped ROS format, each block stores values of multiple columns for some number of rows and therefore there is a single index across all the columns. When storing Col1 and Col2 in a grouped ROS and retrieving position 20,000, for instance, it is not possible to retrieve column Col 1's data without also retrieving column Col 2's data as well.

FIG. 8A is a flow chart 800 of an example technique for writing data to a grouped ROS formatted ROS container. The local catalog (e.g., 302) is accessed to identify which columns of a projection are stored together and their order for a particular node (step 802). Rows from the identified columns are buffered in column buffer 816 (FIG. 8B) until there is a suitable size chunk of data (step 804). For tables with large rows (e.g., many columns, large variable length character strings), this may be a small number of rows. Column-based encoders on then run on each column in the column buffer 816 to generate output blocks (step 806). In some implementations, an encoder consumes column data from the column buffer 816 until it fills up a 64K output encoded output block. Due to differences in column values and column encodings, the same number of rows may produce different numbers of output encode blocks for each column (e.g., an integer column which is run length encoded into a small number of bytes versus a large column of variable length character strings where each value is 64K). The blocks output from the encoding step are written sequentially or packed into an output buffer 818 (FIG. 8B) in a round robin fashion (step 808).

For example, if data from columns 1, 2 and 3 are to be stored together in the grouped ROS formatted ROS container, the output buffer 818 would be filled with a first encoded block for column 1, followed by a first encoded block for column 2, followed by a first encoded block for column 3, followed by a second encoded block for column 1, followed by a second encoded block for column 2, and so on. Due to the variable number of blocks per column, blocks include a header that identifies which column the block is for along with the position range the block includes. In some implementations, the output buffer 818 is compressed 820 (FIG. 8B) with LZO or some other compression technique. The output buffer 1018 is then written as a block to a file for the grouped ROS formatted ROS container (step 810). An entry (e.g., 600 c) into a position index (e.g., 600) for the grouped ROS formatted ROS container is added indicating the starting position of the first block in the buffer and the block's offset within the file (step 812).

FIG. 9A is a flow chart 900 of an example technique for reading data from a grouped ROS formatted ROS container. The local catalog (e.g., 302) is accessed to identify the file used by the grouped ROS formatted ROS container and its index (step 902). The index is then consulted to identify an offset in the file for a chunk of data that contains the block sought based on the position (e.g., row; step 904). The chunk 910 (FIG. 11B) is then read from the file based on its offset and decompressed into buffer 912 (step 906). The chunk 910 is opaque until it is decompressed. Next, the buffer 912's (FIG. 9B) blocks are then scanned by examining block headers until the block for the correct column and position range is found (step 908). In some implementations, multiple reads can use caching for improved performance, e.g., keep the file open, cache the decompressed chunk, and maintain a pointer into the list of blocks for sequential traversals.

In various implementations, a projection can be declared as row optimized, hybrid optimized, or auto optimized. This indicates the type of storage format to be employed for the ROS containers used to store data for the projection. In some implementations, projections are declared auto optimized by default. TABLE 3 below describes the different projection types in additional detail.

TABLE 3 PROJECTION TYPE DESCRIPTION Row Optimized In a row optimized projection, only a grouped ROS format of each ROS container will be stored. This optimizes for row operations such as UPDATE/DELETE or a single row or sparse SELECT, at a minimum space cost, since only one copy of the data will be stored. In further implementations, a ROS container is in row optimized format if the ‘GROUPED’ syntax is used to declare a projection. This is described further below. Auto Optimized In an auto optimized projection, the general policy employed will be that small ROS containers will be stored in the grouped ROS format and large ROS containers will be stored in the column format. In order to establish how many rows constitute a “large” ROS container, the number of bytes per row can be determined. Since the number of bytes per row is not known ahead of time, in some implementations the first ROS container created for every projection will be stored in a grouped ROS format regardless of its size, to be used as a benchmark for future selection. After the first ROS container has been loaded, the statistics collected on the data are used to estimate whether future ROS containers would be considered large or small and the appropriate format is chosen. In further implementations, as the Tuple Mover subsequently merges ROS containers, the Tuple Mover continues to maintain the invariant that there will be at least one such ROS container available in grouped ROS format only at all times to serve as a benchmark for size. In further implementations, auto optimized can be switched to hybrid optimized if numerous row operations are detected and space is available. Note that in some implementations because at least one ROS is stored in grouped ROS format for sizing as described above, there is no need for a “column optimized” option separate from auto optimized. Hybrid Optimized In a hybrid optimized projection, a grouped ROS format will be used for small containers, while redundant grouped ROS and column formats will be selected for large ROS containers. This ensures high performance of UPDATE/DELETE/single row SELECT in many situations, plus high performance aggregation queries on a few columns. Hybrid optimized can be switched to row or auto optimized to save space. In some implementations, as with auto optimized, the first ROS container will be in grouped ROS format to provide information about the physical size of a row.

In some implementations, automatic optimization will employ a policy that requires small ROS containers to be grouped ROS formatted and larger ROS containers to be stored as row-formatted, column-formatted, or both—except that at least one ROS container will be kept in grouped ROS format. In order to establish how many rows constitute a “large” ROS container, the number of bytes/row can be determined. Since the number of bytes/row is not known ahead of time, the first ROS created for every projection can be stored as rows regardless of its size, to be used as a benchmark for future selection. Subsequently, as the Tuple Mover merges ROS containers, it continues to maintain the invariant that there will be at least one such ROS container available in grouped ROS format at all times to serve as a benchmark for size.

In various implementations, a so-called DataTarget operator that writes the data to a WOS or a ROS container is capable of automatically and dynamically switching the destination storage type while it is running. There are at least four advantages of this technique:

-   -   1) If the size of the load is very small, the DataTarget         operator could have been targeting the in-memory WOS as the         storage. However, space in the WOS may become unavailable due to         other concurrent processing. By switching to storing data in a         ROS container mid-operation, the load operation can continue.     -   2) If the size is larger than is appropriate for the WOS, the         compression factor becomes important. However, compression rates         depend on the data. By starting with a grouped ROS formatted ROS         container large enough to even out any transients, the expected         post-compression size of the total load can be estimated. If the         total load is large, the DataTarget operator can switch to a         column formatted ROS container output on the fly to take         advantage of column benefits.     -   3) In some implementations, query performance may be improved by         distributing the data in multiple ROS containers so that data         could be read in parallel.     -   4) In further implementations, a table may be partitioned by a         specific expression to segregate data so that any particular         partition can later be dropped quickly. For example, data may be         partitioned by month so that the oldest month of data can be         dropped easily. In such systems, the Data Target operator can         choose to store data from different partitions in separate         containers. Furthermore, the type of container (WOS, grouped ROS         formatted ROS container, column formatted ROS container) can be         particular to the amount of data involved in each partition.

In a typical implementation of a DataTarget operator that does not dynamically choose the destination container type, the catalog entries (described earlier in reference to FIG. 3) for the storage containers to store the resulting data can be created before the operation starts. During execution, the DataTarget writes data into a set of files. Upon completion, additional changes may be made to the catalog to include information about file sizes, and so on. In some implementation, temporary file names may be initially used and finally updated to permanent files in the database after the operation is complete.

In various implementations of a DataTarget operator that dynamically chooses the destination container type, the DataTarget operator would no longer create the catalog entries apriori. Before execution, the DataTarget would use the catalog to discover which storage locations it should use for any files (possibly with temporary names) it might create, but not make any changes to the catalog. During execution, the DataTarget would write to files just as before. Switching targets merely requires opening a new file and beginning to write. Upon completion, the DataTarget operator would create the requisite catalog entries with the file names of the files it added.

FIG. 10 is a flow chart of an example technique for determining the format (e.g., row or column) of a new ROS container for an auto or hybrid optimized projection based on updating patterns for a projection. If there is an existing ROS container for the projection (step 1002), then the size of a new ROS container based on the size of an existing ROS container is estimated (step 1008). If the size of the uncompressed data (e.g., all columns) is small, for example the expected compressed size is less than a minimum read size threshold (e.g., 1 MB)*expected compression ratio, and there is sufficient free space in the WOS (step 1016), the data is stored in the WOS (step 1020). Otherwise, if the estimated size would exceed 1 megabyte (MB) per column, or some other size threshold, (step 1020), a new column formatted ROS container is used to store the data depending on the optimization (auto or hybrid; step 1012). Otherwise, a grouped ROS formatted ROS container (step 1004) is used to store the rows.

If there is not an existing ROS container for the projection (step 1002) because, for instance, the projection's data is sourced via a network socket or from a query whose output size is difficult to estimate accurately, the size of the projection is estimated based on its uncompressed size and a compression ratio (step 1014). In implementations where data is stored on disk in sorted order and compressed in projections, the data goes through a sort operation before it is compressed and written to disk. A sort operation by its nature cannot complete until it has included every row. Thus, before data actually is written to disk, the uncompressed size of the data is known.

If the size of the uncompressed data (all columns) is small, for example the expected compressed size is less than a minimum read size threshold (e.g., 1 MB) * expected compression ratio, and there is sufficient free space in the WOS (step 1016), the data is stored in the WOS (step 1020). Otherwise, if the estimated size would exceed 1 megabyte (MB) per column, or some other size threshold, (step 1020), a new column formatted ROS container is used to store the data depending on the optimization (auto or hybrid; step 1012). Otherwise, a grouped ROS formatted ROS container (step 1004) is used to store the rows.

If this is a Tuple Mover operation that deletes its sources (e.g., a merge-out operation that merges two ROS containers into a new one and deletes the old containers when it is finished), and it will delete the last grouped ROS formatted ROS container (step 1006), a grouped ROS format is used for the new ROS container (step 1004). Otherwise, the logic continues at step 1008, which is described above.

The next section describes how these formats can be employed as user specified overrides in an illustrative implementation. As described earlier, the system stores data in projections which store subsets of columns of a table in a given sort order and specifying an encoding/compression type for each column. An example of syntax to create a projection is as follows:

create projection p (col1 encoding RLE, col2 encoding RLE,    col3 encoding AUTO) as select col1, col2, col3 from table    order by col1, col2;

In the above example, projection ‘p’ is defined as having columns ‘col1’ and ‘col2’ which are encoded using RLE whereas column ‘col3’ has ‘AUTO’ encoding which specifies that an appropriate encoding type will be chosen automatically. The following example illustrates a projection syntax that can specify row, automatic or hybrid optimized storage for the projection:

  create projection p (col1 encoding RLE, col2 encoding RLE, col3 encoding NONE)   STORAGE HYBRID   as   select col1, col2, col3 from table     order by col1, col2;

In the above example, projection ‘p’ will be stored in hybrid optimized format. Alternatively, ‘ROW’ or ‘AUTO’ could be specified in place of ‘HYBRID’ to indicate that projection ‘p’ will be stored in row or auto optimized storage, respectively. In further implementations, ‘ROW’ does not have to be specified if the projection is created using the ‘GROUPED’ keyword, described next.

In further implementations, the projection syntax is extended to specify that certain columns are to be stored together in a row-oriented format. Projection columns can be defined with the ‘GROUPED’ (<other column (s)>) clause to indicate that they are to be stored with one or more other columns, for instance. Projection columns that have the ‘GROUPED’ keyword specified can be encoded with an additional ‘[RELATIVE TO <col>]’ clause, after the encoding. If ‘RELATIVE TO’ is specified, compression is treated as relative to the GROUPED column.

For example:

create projection p (GROUPED col1 encoding RLE,   col2 encoding DELTAVAL [RELATIVE TO col1],   col3 encoding AUTO)   as   select col1, col2, col3 from table   order by col1, col2;

In the above example, columns ‘col1’ and ‘col2’ will be stored together as a group in the same file in grouped ROS format. Here ‘[RELATIVE TO]’ indicates subtraction, and is applied to integer-based types of equal widths (e.g., int8, date, time, float (treated as integer), and numeric (only if the precision and scale are the same)). Column ‘col2’ will be encoded relative to column ‘col1’. In further implementations, an RLE encoded column that is to be ‘GROUPED’ with another column can be compressed to the same cardinality.

Storage Locations

It is observed that disk drives are typically faster on their outer tracks than on the inner tracks; but logically partitioning disks into two partitions leads to ˜80% of the benefit of careful placement, while partitioning into 3 partitions leads to ˜90% of the benefit. In various implementations, this empirical data and other performance related information about storage devices will be utilized to intelligently place data to achieve better performance. The performance characteristics of the storage such as sustained throughput (megabytes/second) and random seek time (milliseconds) will be recorded, for reads and writes, for instance. These properties can be measured automatically when a storage device is added to the database (henceforth known as a storage locations within the DBMS) and can also be specified by users. Note that multiple storage locations can be created from the same physical disk—for example, a disk could be partitioned into inner and outer tracks and a storage location created out of each one.

In further implementations, storage locations can be tagged based on their use—for instance as DATA (used to store permanent data), TEMP (used to store temporary data), ARCHIVE (used to store infrequently accessed archived data), and so on. Components within the system 200 can explicitly request a storage location of a particular type. For example a Sort Operator process within the Execution Engine 216 may ask for a TEMP storage location to keep its intermediate sorted blocks of data. Similarly, when deleting old data from the database, users can request that data to be archived to a storage location of type ARCHIVE, which could be a high-capacity but low-performance disk array. In yet further implementations, as solid state disks become more prevalent, the tags could be used to advise the system 200 of the specific write characteristics of the solid state disks, which could be used to optimize the database storage and input/output algorithms.

In an illustrative implementation, storage locations are added to the database by using a function call known as ‘add_location’ as follows:

select add_location(‘<path>’,‘<node>’, ‘DATA|TEMP|ARCHIVE’);

This function call tags the storage location according to its projected usage as a storage for permanent, temporary or archive data. The ‘<path>’ parameter refers to the actual file or directory where data will be stored. The ‘<node>’ parameter refers to the database node where the additional storage is being added. In some implementations, the ‘<node>’ parameter is not required if, for example, the database is implemented on a single node rather than a cluster. A storage location can also have more than one tag associated with it—by default, a storage location can be used for both ‘DATA’ and ‘TEMP’ storage. In some implementations, there is at least one ‘DATA’ and one ‘TEMP’ location for a system 400, possibly the same location serving both uses.

The tag for a storage location can be altered if the storage location is being re-purposed. For example, older disks may be re-purposed to serve as archive storage in future.

select alter_storage_use(‘<path>’,‘<node>’, ‘DATA|TEMP|ARCHIVE’);

Once tagged, the storage locations of a specific type can be requested by various operations.

In further implementations, performance characteristics of storage locations can be measured automatically (or set by users) and saved by the DBMS by invoking a routine follows.

select measure_storage_performance(‘<path>’,‘<node>’); or set_storage_performance(‘<node>’,‘<path>’,‘<throughput>’,    ‘<avg latency>’);

In the above functions, the ‘<path>’ and ‘<node>’ parameters have the same meanings as before. The ‘<throughput>’ and ‘<avg latency>’ parameters refer to the sustained throughput and average disk seek time for the storage device underlying the given location. From the throughput and latency, a speed number is calculated based on an anticipated pattern of 1 MB random I/Os, thus add the latency to the time to transfer 1 MB at given throughput. Other ways of determining a speed number are possible. Each storage location is thus associated with a single speed number that represents its performance characteristics.

In further implementations, the projection syntax is extended to include the following parameters:

<name> [ENCODING <type>] [SPEED n]

to indicate preferred speed of a storage location to use for a column. For example:

create projection (col1 [ENCODING <type>] [SPEED n], ...) as ... order by ...

In some implementations, a ‘SPEED’ is specified as an integer, relative to the default priority of 1000. Columns with smaller numbers will be placed on slower disks, higher numbers on faster disks. In some implementations, if the ‘SPEED’ parameter is not specified, columns in the sort order (order by) will be given higher priority (e.g., numbers>1000), with the last column in the sort order receiving 1001, and the first column in the sort order given the highest priority (1000+# of sort columns). Remaining columns will be given numbers starting at 1000 and working down (never to go below 1). As a result, in the default configuration, any multi-column projection can have a mix of speed settings associated with the columns.

FIG. 11 is a flow chart of an example technique for assigning columns to storage. Initially, a set of storage locations to be used is identified excluding locations that are not to be used for the DATA access pattern, or locations that do not match the speed setting of the projection or data partition. (step 1102). The set of locations are then ordered by speed—fastest first—using the calculated SPEED number, as described earlier (step 1104). In some implementations, if the set of columns is assigned to be stored in a grouped ROS format, the speed setting used can be the average of all the columns (for a conservative placement policy) or the fastest of all the columns (for an aggressive placement policy). The columns are ordered according to speed suggestions specified in the projection definition, most performance first (step 1106). In various implementations, the columns and locations are traversed in order, placing as many columns on each storage location as indicated by the free space remaining in the location (step 1108). Thus a disk with N % of the total free space will store ˜N % of the column data, for example. In further implementations, the placement algorithm can be modified dynamically by collecting usage statistics on the column based on queries asked in the system. Also, the Tuple Mover can provide more accurate space usage statistics for the columns which can then be used to improve the efficacy of step 1106.

Tuple Mover

At any given time there may be multiple ROS containers on disk. A policy-driven Tuple Mover 212 places upper bounds on the number and sizes of ROS containers in the system and the resources consumed by the system's operations. The Tuple Mover is responsible for so-called move-out and merge-out operations. The move-out operation moves data out of the WOS to a new ROS container, determines the appropriate format for the ROS container (see FIG. 10 above), and converts the data from its WOS format to the ROS container's format as part of the operation. The Tuple Mover 212 determines when to perform the move-out operation based on trading off moving data out frequently (which would lead to many small ROS containers that will have to merged later) and moving data out infrequently (which could result in data loss if there is a system failure).

In various implementations, the Tuple Mover 212 performs the move-out operation periodically based on a configurable time period and moves WOS data to a ROS container if the data exceeds a certain configurable age or size threshold. The maximum age of the WOS data is configurable as a policy setting. If the WOS for any projection has data older than that suggested by the policy, for instance, the oldest WOS data will be scheduled for move-out. Once all WOS data have been moved based on age, additional WOS data can be selected for move-out if the total WOS size still exceeds the policy threshold. The move-out operation can run concurrently with the merge-out operation.

The merge-out operation merges two or more ROS containers to a new ROS container that has a format (grouped ROS or column) determined by the operation (see FIG. 10 above). In some implementations, an n-way merge-out corresponds to the number of ROS containers will be merged in a single merge-out operation. An n-way merge-out operation to merge n ROS containers can be implemented as a sequence of two-way merges but this may be less efficient because the data will be merged multiple times. Generally speaking, the number of ROS containers merged depends on how much memory is available for the merge-out operation, for example. The merge-out operation can be a resource intensive operation and can take a long time to complete. In general, managing the number and sizes of ROS containers in the system can be complex and can have an impact on query performance. In some implementations, there would be a small number of ROS containers such that the node 202 is kept busy by processing queries by accessing ROS containers rather than by merging them.

In various implementations, a policy for the merge-out operation satisfies one or more of the following goals:

-   -   Provide an upper bound on the number of ROS container files         present in the system before the Tuple Mover takes action.     -   Provide an upper bound on an amount of resources that are         engaged in merge operations.     -   Merge files of similar size, primarily for efficiency.     -   Attempt to limit the number of times a row of data will undergo         the merge process, again for efficiency (e.g., minimize CPU and         disk resources used per tuple).     -   Attempt to perform small merges quickly, as these are “low         hanging fruit”.

In different implementations, the parameters detailed in TABLE 4 below control the merge-out operation. Other parameters are possible, however.

TABLE 4 PARAMETER DESCRIPTION Memory Budget An amount of memory (e.g., random access memory) allocated for some task (e.g., a maximum number of ROS containers). Maximum ROS A maximum number of ROS containers that can be kept per Containers Per projection; this is generally based on the memory budget. Projection (“MCP”) ROS Container Size The sum of file sizes for all files that constitute a ROS container. Stratum A category of ROS containers based on a ROS container size range. ROS containers are categorized into contiguous, non- overlapping strata based on their size. A stratum is defined by a (minimum size, maximum size) pair. See FIG. 12. Stratum Height For a stratum, the quotient obtained by dividing the largest ROS (“StratumHeight”) container size defined in the stratum size range by the smallest ROS container size in the range. In some implementations, the same height is used for all strata; so that for stratum n >= 1 the maximum size is height * minimum size[n], or height * maximum size[n − 1], for instance. Maximum ROS The largest ROS container size one would ever expect to see. By Container Size way of illustration, an obvious maximum is the total size of all disk volumes, which is likely smaller than a logical max of 2⁶⁴ bytes per column. Negligible ROS A lower limit of a ROS container's size, where the cost of opening Container Size the ROS container's files (which involves disk seeks and so on) approximates the cost of scanning the ROS containers. In such cases, there is very little advantage to merging the ROS container with ROS containers of similar size, as long as the other ROS containers are also negligible. In some implementations, the range of sizes for stratum 0 is from a minimum of 1 byte to a maximum of the negligible ROS container size (provided that there is enough space to use the strata algorithm at all). In further implementations, the negligible ROS size is 1 MB per column. Number of Strata The number of strata in the categorization scheme, including (“StratumCount”) stratum 0. See FIG. 12. Maximum ROS The number of ROS containers present in a stratum before the containers per Stratum Tuple Mover 212 will attempt to merge the ROS containers in the (“MCS”) stratum. See FIG. 12.

The merge-out operation categorizes ROS containers into “strata” based on their size, as shown FIG. 12. Generally speaking, the merge-out operation determines, for a given projection, which set of ROS containers to merge. To assess a projection for merge-out, the Tuple Mover 212 determines the strata for the projection and the MCS. If any stratum has the MCS (or more), a merge-out operation will be indicated. In some implementations, the merge-out operation will be for the smallest stratum that is eligible, and will operate on the ROS containers with the smallest start epochs (using end epoch and size to break ties) until there are no more than MCS containers.

FIGS. 13A-B illustrate an example technique 1300 for determining the strata for a projection. In step 1302, the memory budget from TABLE 4 is used to compute the MCP. In some implementations, the space per ROS container is the number columns in the projection (“CIP”)×2 megabytes. Other ways of calculating the space per ROS container are possible. In further implementations, the number of projection columns includes a hidden internal epoch column. The epoch is a system-maintained “clock” that advances automatically at regular intervals, e.g., 5 minutes. Each transaction is stamped with the current epoch when the transaction commits. All transactions that commit within the same epoch will have all inserted data rows with that epoch value for the epoch column. Similarly, when data is deleted, the current epoch is recorded with the positions in the delete vectors. This is used to retrieve results as of a time in the past (e.g., historical queries). By way of illustration, a user can query the RDBM for results as they were at 10 AM or epoch 55.

In yet further implementations, space may be set aside for one or more so-called delete vector files. In these implementations data is not deleted immediately when a SQL DELETE statement is executed. Instead, deletes are implemented by storing the list of positions (rows) within the ROS container that are deleted—the data structure which stores these deleted positions is referred to as a delete vector. Each ROS container may been associated with one or more delete vectors that represent deleted positions within that container. The Tuple Mover 212 will remove the deleted records when it rewrites the ROS container as part of its merge-out operation.

The MCP is defined in some implementations as, for example:

${MCP} = \frac{MemoryBudget}{{{CIP} \times 2{MB}} + {DeleteVectorSpace}}$

If MCP is less than two, then MCP is set to 2.

In step 1304, the strata range (“StrataRange”) is determined as MRS/NRS. In some implementations, the NRS is computed as 1 megabyte times the number of projection columns. Other ways of determining the NRS are possible. In some implementations, the MRS is computed as half of the total disk space in data storage locations associated with the RDBMS. Other ways of determining the MRS are possible.

In step 1306, the number of stratums (“StratumCount”) and the MCS are determined. If the MCP as determined above is <4 (step 1308), the StratumCount is set to 1, the MCS is set to the MCP and the StratumHeight is set to the MRS (step 1310) and the technique completes. Otherwise, the StratumHeight is determined assuming that the MCS is 2 and that the Stratum Count is equal to MCP/2 (step 1312). In various implementations, the StratumHeight is determined using the following formula since all strata have the same height: StratumHeight^(StratumCount−1)=StrataRange

Use of StratumCount−1 is because one stratum is for the negligibly-sized ROS containers (see FIG. 12). This can be rewritten as: StrataHeight=^(StratumCount−1)√{square root over (StrataRange)}

If StrataHeight is greater or equal to 2 (step 1314), then two-way merges are the best way to merge items of approximately equal size. Therefore, in step 1316 MCS is set to 2.

If StrataHeight is less then 2 (step 1314), then it is possible that more than two ROS containers can be used per stratum. In step 1318, a maximum value for the MCS is determined where the following inequality holds:

$\left( {0.9 \times {MCS}} \right)^{({\frac{MCP}{MCS} - 1})} \geq {StrataRange}$

(Here, the 0.9 expresses the possibility that the resulting ROS container may be slightly smaller than the sum of the ROS containers being merged. Other constants are possible.) This inequality can be solved using iteration, for example, starting with MCS=2 and working up until the inequality is broken. Other ways of solving the inequality are possible. In step 1320, StrataHeight is determined assuming the value of MCS determined in step 1318 and assuming StrataCount is equal to MCP/MCS.

Embodiments of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, or in executing computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer-readable medium for execution by, or to control the operation of, data processing apparatus. The computer-readable medium can be one or more machine-readable storage devices, machine-readable storage substrates, memory devices, compositions of matter, or a combination of one or more of them. The term “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio player, a Global Positioning System (GPS) receiver, to name just a few. Computer-readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input.

Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described is this specification, or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), e.g., the Internet.

The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other.

While this specification contains many specifics, these should not be construed as limitations on the scope of the implementation or of what may be claimed, but rather as descriptions of features specific to particular embodiments of the implementation. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

Thus, particular embodiments of the implementation have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. 

What is claimed is:
 1. A computer-implemented method comprising: determining a size of projection data to be stored in a read-optimized store (ROS) container, wherein the projection data includes rows and columns of data; determining whether the size of the projection data is greater than a predetermined threshold; in response to a determination that the size of the projection data is greater than the predetermined threshold, storing the projection data in the ROS container in a column format, wherein, in the column format, each column of the projection data is stored in a separate file; and in response to a determination that the size of the projection data is less than the predetermined threshold, storing the projection data in the ROS container in a grouped ROS format, wherein, in the grouped ROS format, each of a plurality of blocks formed of respective groups of rows and columns of the projection data is stored in a separate file wherein storing the projection data in the grouped ROS format includes: selecting a plurality of columns of the projection data, and storing rows of the selected columns as blocks of data in the ROS container until all of the rows of the selected columns are stored in the ROS container.
 2. The method of claim 1, wherein the projection data is stored persistently.
 3. The method of claim 1, wherein the projection data is to be stored in the ROS container when the projection data exceeds an age or an aggregate size.
 4. The method of claim 1, further comprising updating the projection data in a write-optimized store before storing the projection data in the ROS container.
 5. The method of claim 1, further including selecting an order of the columns in the selected subset of columns to be stored together.
 6. The method of claim 5 in which each row's first columns are stored together in a single file with at least one other row's first columns.
 7. The method of claim 1, wherein storing the projection data in the column format comprises: selecting one column of the projection data, storing a subset of rows of data of the selected column as a block of data in the ROS container, and storing additional subsets of rows of data of the selected column as additional blocks of data in the ROS container until all of the rows of data of the selected column are stored in the ROS container.
 8. The method of claim 7, wherein the blocks of data of the selected column are stored in separate files.
 9. The method of claim 1, further comprising storing rows of the projection data for columns in the grouped ROS format based on a frequency of use of the columns of the rows.
 10. The method of claim 1, further comprising updating the projection data in response to a request to write data to the projection.
 11. The method of claim 10, wherein the updating is performed in response to processing a database operator to write data to storage.
 12. A computer-implemented method comprising: selecting at least two different storage formats to store data of a database projection based on patterns of updating data of the projection, wherein the at least two different storage formats include a column format and a grouped read-optimized store (ROS) format, wherein the column format is to store each column of the data in a separate file, and the grouped ROS format is to store each of a plurality of blocks formed of respective groups of rows and columns of the data is stored in a separate file; updating the projection data based on the patterns using the selected storage formats; determining whether a size of the updated projection data is greater than a predetermined threshold; in response to a determination that the size of the updated projection data is greater than the predetermined threshold, storing the updated projection data in the column format; and in response to a determination that the size of the updated projection data is less than the predetermined threshold, storing the updated projection data in the grouped ROS format, wherein storing the updated projection data in the grouped ROS format includes: selecting columns of the updated projection data to be stored together, and storing rows of the selected columns as blocks of data in the store until all of the rows of the selected columns are stored in the store.
 13. The method of claim 12, wherein the storage formats further comprise a write-optimized storage format.
 14. The method of claim 12, further comprising storing the updated projection data persistently.
 15. The method of claim 12, wherein the projection data is stored at a time based on at least one of an age or an aggregate size of the of the projection data.
 16. The method of claim 12, further comprising storing the updated projection data in a write-optimized store.
 17. The method of claim 12, further comprising: determining the size of the updated projection data by estimating a storage size of rows of the updated projection data based on a size of previously stored projection rows; and storing the rows of the updated projection data in the grouped read-optimized store (ROS) format or in the column format based on the estimated storage size.
 18. The method of claim 12, wherein each of the blocks of data is stored in a single file.
 19. The method of claim 12, wherein storing the updated projection data in the column format includes: selecting one column of the updated projection data, storing a plurality of rows of the selected column in a file, and storing additional rows of the selected column in the file until all of the rows of the selected column are stored in the file.
 20. The method of claim 12, further comprising storing rows of the updated projection data in the grouped read-optimized store (ROS) format based on a frequency of use of columns of the rows.
 21. The method of claim 12 wherein updating is performed in response to processing a database operator to write data to storage.
 22. A non-transitory computer readable medium on which is stored a set of instructions that, when executed, causes a device to: determine a size of projection data to be stored in a read-optimized store (ROS) container, wherein the projection data includes rows and columns of data; determine whether the size of the projection data is greater than a predetermined threshold; in response to a determination that the size of the projection data is greater than the predetermined threshold, store the projection data in the ROS container in a column format, wherein, in the column format, each column of the projection data is stored in a separate file; and in response to a determination that the size of the projection data is less than the predetermined threshold, store the projection data in the ROS container in the grouped ROS format, wherein, in the grouped ROS format, each block of rows and columns of the projection data is stored in a separate file, wherein to store the projection data in the grouped ROS format, the set of instructions is to cause the device to: select a plurality of columns of the projection data, and store rows of the selected columns as blocks of data in the ROS container until all of the rows of the selected columns are stored in the ROS container.
 23. A computer-implemented method comprising: selecting a plurality of candidate containers to store projection data; identifying a plurality of containers to merge in the candidate containers; selecting storage formats to store the projection data, including: determining whether a size of the projection data is greater than a predetermined threshold; in response to a determination that the size of the projection data is greater than the predetermined threshold, selecting a column format to store the projection data, wherein, in the column format, each column of the projection data is stored in a separate file; and in response to a determination that the size of the new rows is less than the predetermined threshold, selecting a grouped read-optimized store (ROS) format to store the projection data, wherein, in the grouped ROS format, each of a plurality of blocks formed of respective groups of rows and columns of the projection data is stored in a separate file; and merging the identified containers into a new container according to the selected storage formats, wherein selecting the storage formats is based on an assessment of existing storage allocated to the projection data such that some but not all of projection data is stored in the grouped ROS format, wherein selecting the grouped ROS format to store the projection data includes: selecting a plurality of columns of the projection data to be stored together, and storing rows of the selected columns as blocks of data in the store until all of the rows of the selected columns are stored in the store.
 24. The method of claim 23 wherein the selected candidate containers are in a common size range.
 25. The method of claim 23 wherein the identified containers are each associated with a start epoch that is smaller than the remaining candidate containers. 